However, the photographic markings on passerine eggs have been suggested to reduce brittleness by acting as a solid state lubricant.[1] If insufficient calcium is available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil.[2]

For the same reason, later eggs in a clutch are more spotted than early ones, as the female's store of calcium is depleted.

Birds which build in trees generally have blue or greenish eggs, either spotted or unspotted, while birds that build in bushes or near or on the ground are likely to lay speckled eggs.

The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting the gene responsible for pigmentation is on the female-specific W chromosome (female birds are WZ, males are ZZ).

Color was once thought to be applied to the shell immediately before laying, but this research shows coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral.

In species such as the common guillemot, which nest in large groups, each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed.

The shape of eggs varies considerably across bird species, ranging from near-spherical (such as those of the little bee-eater) to highly pyriform or conical (such as those of the common murre) with the familiar shape of the chicken egg lying in between. Early scientific investigators of egg shape [3] suggested that the oval shape of eggs was caused by the egg being forced through the oviduct by peristalsis. In this often-repeated [4][5][6] but incorrect theory of egg shape formation, the contraction and relaxation of the muscles which push the egg down the oviduct cause the spherical egg membrane to distort slightly into an ovoid shape, with the blunt end caudal (i.e. furthest down the oviduct and closest to the cloaca). The calcification of the egg in the shell gland/uterus then fixes it in this shape, and the egg is laid with the blunt end appearing first (for a diagram showing the different sections of the bird oviduct, see here). However, this theory has been refuted by studies of egg shell formation in a number of bird species using techniques such as X-ray photography [7][8] which have demonstrated that egg shape is determined in the oviduct isthmus (before shell calcification) with the pointed end caudal (furthest down the oviduct). These observations cannot be explained by peristalsis. It has been proposed [9] that the egg acquires its shape (with the pointed egg caudal) as it is forced through the narrow isthmus, but this assertion has not been thoroughly verified.

Cliff-nesting birds often have highly conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly spherical eggs.

The shape has biological significance. A pointed egg will tend to sit on its side, with the big end tipped upward. The big end contains the air sac and its shell is pierced by a higher density of pores than the more pointed end. Tipping the big end upwards improves oxygen flow to the large head, with the physiologically demanding eyes and brain, that develops in the big end while the tail develops at the more pointed end.

In a 2017 publication in the journal science, mathematical modeling of 50,000 bird eggs data showed that bird egg shape is a product of flight adaptations and not the outcome of nesting conditions or a bird's life history. [10] A strong correlation was found between egg shape and flight ability on broad taxonomic scales, such that birds engaging in high powered flights usually maximize egg size by having elliptical shaped eggs while maintaining a streamlined body plan.[10]

Egg size tends to be proportional to the size of the adult bird,[citation needed] from the half gram egg of the bee hummingbird to the 1.5 kg egg of the ostrich. Kiwis have disproportionately large eggs, up to 20% of the female's body weight.[11] This evolutionary trait results in kiwi chicks that can emerge from the nest days after hatching, ready to start foraging for food.

Brood parasitism occurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include the cowbirds and many Old World cuckoos.